A patterned beam of EUV light can be used to expose a resist coated substrate, such as a silicon wafer, to produce extremely small features in the substrate. Extreme ultraviolet light (also sometimes referred to as soft x-rays) is generally defined as electromagnetic radiation having wavelengths in the range of about 5-100 nm. One particular wavelength of interest for photolithography occurs at 13.5 nm and efforts are currently underway to produce light in the range of 13.5 nm+/−2% which is commonly referred to as “in band EUV” for 13.5 nm systems.
Methods to produce EUV light include, but are not necessarily limited to, converting a source material into a plasma state that has a chemical element with an emission line in the EUV range. These elements can include, but are not necessarily limited to xenon, lithium and tin.
In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a source material, for example in the form of a droplet, stream or wire, with a laser beam. In another method, often termed discharge produced plasma (“DPP”), the required plasma can be generated by positioning source material having an EUV emission line between a pair of electrodes and causing an electrical discharge to occur between the electrodes.
As indicated above, one technique to produce EUV light involves irradiating a source material. In this regard, CO2 lasers outputting light at infra-red wavelengths, i.e., wavelengths in the range of about 9 μm to 11 μm, may present certain advantages as a so-called ‘drive’ laser irradiating a source material in an LPP process. This may be especially true for certain source materials, for example, source materials containing tin. One advantage may include the ability to produce a relatively high conversion efficiency between the drive laser input power and the output EUV power.
Generally, for an LPP light source, EUV output power scales with the drive laser power. It has been suggested to employ an Oscillator—Amplifier arrangement to produce relatively high power laser pulses used in the LPP process. For example, in some arrangements, a multi-chamber amplifier having a one-pass small signal gain in the order of 1×105 or more may be seeded with the output of a somewhat fragile oscillator (seed laser) which may include one or more relatively sensitive optics. In fact, for some setups, the amplifier gain is so high that a polarization discriminating optical isolator, which may, for example, stop about 90-99 percent of back-propagating light, may be insufficient to protect the oscillator from damage. Meeting future demands for increased EUV output with an Oscillator—Amplifier arrangement would require an even larger amplifier, which in turn, would even further endanger fragile oscillator optics. As used herein, the term “seed laser” and its derivatives means a laser, the output of which is injected into some amplifier, or another laser.
It has also been previously suggested to irradiate droplets with a laser beam produced by an optical amplifier that is not seeded by a seed laser. FIG. 1 shows an arrangement in which an EUV light source has an optical amplifier 2 having a chain of amplifier chambers 2a-c arranged in series. In use, a droplet of target material 3 is placed on a trajectory passing through a beam path 4 extending through the amplifier. When the droplet reaches the beam path 4, some photons on the beam path are reflected through the amplifier chain between the droplet and optic 5. This then produces an amplified beam which irradiates the droplet and produces EUV light emitting plasma. For this process, an optical amplifier having a relatively high gain is typically employed. This high gain, however, may be problematic in some cases. Specifically, so-called ‘self-lasing’ may occur before the droplet reaches the beam path 4 due to reflections and/or diffraction from vessel walls, debris from a previously irradiated droplet, optical mounts in the amplifier chain, or in the optics between the amplifier chain and irradiation site, a gate valve separating the EUV light source from a downstream exposure tool, other structures in the chamber and/or on-axis reflections from a drive laser focusing lens.
The amount of self lasing is proportional to amplifier gain and can undesirably deplete amplifier gain needed to produce a target irradiation beam. Meeting future demands for increased EUV output with the unseeded amplifier arrangement shown in FIG. 1 would require an amplifier with even larger gain, which in turn, would cause increased self lasing.
With the above in mind, Applicant discloses Drive Laser Delivery Systems for EUV Light Source.